Gas turbine engine turbine nozzle bifurcated impingement baffle

ABSTRACT

A turbine nozzle bifurcated impingement baffle includes axially forward and aft chambers with a gap therebetween, impingement holes through forward and aft baffle walls of the forward and aft chambers respectively, and a plenum chamber in fluid flow communication with the forward and aft chambers. An exemplary embodiment of the bifurcated impingement baffle further includes a single cooling air inlet to the plenum chamber enclosed within a plenum chamber enclosure the plenum chamber. A sealing plate is mounted between and sealed to the plenum chamber enclosure and the forward and aft chambers. The sealing plate has forward and aft inlet apertures disposed between the plenum chamber and the forward and aft chambers respectively. Forward and aft end plates cap radially inner ends of the forward and aft chambers. An outlet aperture in the forward end plate has an interstage seal cavity feed tube disposed therethrough.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to gas turbine engines turbine nozzles segmentsand, particularly, for such segments having hollow vanes or airfoilswith a cavity for receiving a cooling air distributing baffle.

2. Description of Related Art

In a typical gas turbine engine, air is compressed in a compressor andmixed with fuel and ignited in a combustor for generating hot combustiongases. The gases flow downstream through a high pressure turbine (HPT)having one or more stages including one or more HPT turbine nozzles androws of HPT rotor blades. The gases then flow to a low pressure turbine(LPT) which typically includes multi-stages with respective LPT turbinenozzles and LPI rotor blades.

The HPT turbine nozzle includes a plurality of circumferentially spacedapart stationary hollow nozzle vanes supported between radially outerand inner bands. Typically, a single chamber impingement baffle isinserted in each hollow airfoil to supply cooling air to the airfoil.Each baffle can be fed through a single spoolie located radiallyoutwardly of the outer band of the nozzle.

The turbine rotor stage includes a plurality of circumferentially spacedapart rotor blades extending radially outwardly from a rotor disk whichcarries torque developed during operation. The HPT nozzles are typicallyformed in arcuate segments having two or more hollow vanes joinedbetween corresponding segments of the outer and inner bands. Each nozzlesegment is typically supported at its radially outer end by a flangebolted to an annular outer casing. Each vane has a cooled hollow airfoildisposed between radially inner and outer band panels which form theinner and outer bands. The airfoil, inner and outer band portions,flange portion, and intake duct are typically cast together such thateach vane is a single casting. The vanes are brazed together alonginterfaces of the flange segments, inner band panels, and outer bandpanels to form the nozzle segment. Two or more airfoils may also be casttogether in a single vane or nozzle segment.

Certain two-stage turbines have a cantilevered second stage nozzlemounted and cantilevered from the outer band. There is little or noaccess between first and second stage rotor disks to secure the segmentat the inner band. Typical second stage nozzles are configured withmultiple airfoil or vane segments. Two vane designs, referred to as adoublets, are a very common design. Doublets offer performanceadvantages in reducing split-line leakage flow between vane segments.However, the longer chord length of the outer band and mountingstructure compromises the durability of the doublet. The longer chordlength causes an increase of chording stresses due to the temperaturegradient through the band and increased non-uniformity of airfoilstresses. The box structure of a vane doublet also contributes to unevenstresses in the segment. The trailing vane of a doublet typically seessignificantly higher stresses which limits the life of the segment.

It is highly desirable to have a turbine nozzle segment that providesaccess for a cooling air supply between first and second stage rotordisks and the ability to be cantilever mounted from the outer band. Itis also desirable to have turbine nozzle segments that avoid reductionin the durability of multiple vane segments due to longer chord lengthof the outer band and mounting structure. It is also desirable to haveturbine nozzle segments that avoid increase of chording stresses due totemperature gradient through the band and increased non-uniformity ofairfoil stresses due to longer chord length of the multiple vanesegments. It is also desirable to have turbine nozzle segments thatavoid increase of stresses in a trailing vane of a doublet or othermultiple vane segment which limits the life of the segment.

A single vane segment, referred to as a singlet, is disclosed in U.S.patent application Ser. No. 10/375,585 and has a bifurcating stiffeningrib which divides a bifurcated cavity into forward and aft cavitiesrequiring two baffles. A cantilevered mount for turbine nozzle segmentsis disclosed in U.S. patent application Ser. No. 10/375,441. Due to thecantilevered design of the second stage nozzle, there is insufficientaccess between the first and second stage rotor disks to feed coolingair from a radially inner side of the nozzle. Thus, both baffles must befed cooling air from the outer side of the nozzle. Thus, it is desirableto have, a baffle for a bifurcated cavity that is able to be fed coolingair from the outer side of the nozzle to both the forward and aftcavities. It is desirable to have a baffle able to feed cooling air fromthe baffle to an interstage seal cavity.

SUMMARY OF THE INVENTION

A bifurcated impingement baffle for use in a gas turbine engine turbinenozzle includes axially forward and aft chambers with a gaptherebetween, impingement holes through forward and aft baffle walls ofthe forward and aft chambers respectively, and a plenum chamber in fluidflow communication with the forward and aft chambers. An exemplaryembodiment of the bifurcated impingement baffle further includes asingle cooling air inlet to the plenum chamber. A plenum chamberenclosure encloses the plenum chamber. A sealing plate is mountedbetween and sealed to the plenum chamber enclosure and the forward andaft chambers. The sealing plate has forward and aft inlet aperturesdisposed between the plenum chamber and the forward and aft chambersrespectively. Forward and aft end plates cap radially inner ends of theforward and aft chambers. An outlet aperture in the forward end platehas an interstage seal cavity feed tube disposed therethrough. A tubeend cap seals the interstage seal cavity feed tube. The interstage sealcavity feed tube feeds cooling air from the baffle to an interstage sealcavity.

The bifurcated impingement baffle is designed for use in a gas turbineengine turbine nozzle and more particularly in a turbine nozzle segmenthaving at least one hollow airfoil extending radially between radiallyouter and inner band segments. The airfoil has an airfoil wall withpressure and suction sides extending axially between leading andtrailing edges of the airfoil and surrounding a bifurcated cavity. Abifurcating rib extending through the bifurcated cavity divides thebifurcated cavity into forward and aft cavities. The forward and aftchambers are disposed in the forward and aft cavities respectively.

The bifurcated impingement baffle allows cooling air to be fed from theouter side of the nozzle. It also allows a single inlet and spoolie tobe used to feed the impingement baffle. The bifurcated impingementbaffle also provides for feeding cooling air from the baffle to aninterstage seal cavity. The bifurcated impingement baffle allows the useof a singlet nozzle which is very durable. The baffle's plenum fitswithin the space constraints of the nozzle and operates with only onespoolie which reduces leakage and complexity.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features of the invention are explainedin the following description, taken in connection with the accompanyingdrawings where:

FIG. 1 is a cross-sectional view illustration of a section of a gasturbine engine high pressure turbine second stage turbine nozzle with avane airfoil having a hollow interior bifurcated by a bifurcating ribextending between pressure and suction sides of the airfoil.

FIG. 2 is a perspective view illustration of the second stage turbinenozzle segment illustrated in FIG. 1.

FIG. 3 is a cross-sectional view illustration of the second stageturbine nozzle segment illustrated in FIG. 1.

FIG. 4 is a radially outwardly looking perspective view illustration ofthe airfoil illustrated in FIG. 2.

FIG. 5 is a cross-sectional view illustration of the bifurcating rib anda stiffening rib through 5—5 in FIG. 2.

FIG. 6 is a perspective view illustration of a two chamber impingementbaffle disposed in the bifurcated interior of the hollow vaneillustrated in FIGS. 2 and 3.

FIG. 7 is an exploded perspective view illustration of the impingementbaffle illustrated in FIG. 6.

FIG. 8 is a perspective view illustration of a sector of the secondstage turbine nozzle illustrated in FIG. 1.

FIG. 9 is an enlarged perspective view illustration of aft support ofthe second stage turbine nozzle segments illustrated in FIG. 8.

FIG. 10 is an enlarged perspective view illustration of anti-rotationand tangential load stops on one of the second stage turbine nozzlesegments illustrated in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

Illustrated in FIG. 1 is an exemplary second stage turbine nozzle 4 ofan aircraft gas turbine engine high pressure turbine 2. The nozzle 4 iscircumscribed about a longitudinal or axial centerline axis 6 andincludes an annular casing 14 from which a plurality of nozzle segments10 are cantilevered mounted. The nozzle segment 10 is disposed betweenan immediately upstream row of high pressure turbine first stage rotorblades 18 and an immediately downstream row of turbine second stagerotor blades 9. First and second shrouds 97 and 99 encircle the firstand second stage turbine rotor blades 18 and 9 and are supported byfirst and second shroud supports 77 and 79, respectively, which dependradially inwardly from and are connected to the annular casing 14. Thenozzle segments 10 are hooked by forward hooks 107 to the first shroudsupports 77 and are cantilevered from the second shroud supports 79.

FIG. 2 illustrates one of the nozzle segments 10 including a singlehollow vane airfoil 28 that extends radially between and is integrallyjoined to radially outer and inner band segments 24 and 26 which arearcuate in shape. The airfoil 28 has pressure and suction sides 22 and23 and extends axially between leading and trailing edges LE and TE.Each nozzle segment 10 has only one airfoil 28 which is located betweencircumferentially spaced apart pressure and suction side edges 33 and 35of the radially outer and inner band segments 24 and 26. The nozzlesegment 10 is illustrated as an integral and made from a unitaryone-piece casting. The hollow airfoil 28 has an airfoil wall 29surrounding a bifurcated cavity 37 formed by a bifurcating rib 39extending between the pressure and suction sides 22 and 23 and splittingthe cavity into axially forward and aft cavities 41 and 43.

A stiffening rib 60 extends radially outwardly from a radially outersurface 62 of the outer band segment 24 and extends axially andcircumferentially from a pressure side forward corner 64 of the outerband segment 24 to the bifurcating rib 39. The stiffening rib 60 and thebifurcating rib 39 run in a direction 63 of the resultant gas loads onthe airfoil 28 and provides significant stiffness to the nozzle segment10. The stiffening rib 60 is axially and circumferentially aligned withthe bifurcating rib 39. The stiffening rib 60 and the bifurcating rib 39may be viewed as being substantially co-planar. This provides stiffnessto the nozzle segment 10 and reduces the deflections thereof. Thestiffening rib 60 also provides additional safety in the event ofairfoil cracking.

The forward hook 107 extends forwardly from the outer band segment 24. Aradially inner airfoil fillet 89 runs around a junction between theairfoil 28 and the inner band segment 26. Illustrated in FIGS. 4 and 5,is a radially outer airfoil fillet 91 running around a junction betweenthe airfoil 28 and the outer band segment 24. The outer airfoil fillet91 has a first enlarged portion 90 under the forward hook 107 tominimize mechanical stresses in this region. The outer airfoil fillet 91has a second enlarged portion 93 where the bifurcating rib 39 intersectsthe suction side 23 of the airfoil 28 to minimize mechanical stresses inthis region.

The bifurcating rib 39 running down the airfoil 28 provides a number ofbenefits including helping to prevent fatigue cracking around theairfoil fillet from propagating across the rib so the airfoil will notfail. It allows the airfoil wall 29 to be thinner than with aconventional single cavity design because of increased support from thebifurcating rib 39 which makes the airfoil wall 29 along the pressureside 22 of the airfoil 28 less prone to ballooning. The bifurcating rib39 is angled with respect to the pressure and suction sides 22 and 23 ofthe airfoil 28 to allow an impingement-cooling bifurcated insert orbaffle to be assembled into the forward and aft cavities 41 and 43illustrated in FIGS. 2 and 3.

The bifurcated cavity 37 makes it necessary to have twoimpingement-cooling baffles. Due to the cantilevered design of thenozzle segment 10, there is no access between the first and second stagerotor blades 18 and 9 to feed cooling air from a radially inner side 100of the nozzle segment 10. Thus, both baffles must be fed from a radiallyouter side 104 side of the nozzle segment. This presents challengesbecause there is limited space on the outer side of the nozzle. Twobaffles could be used if fed from independent spoolies, but thisconfiguration is complex to assemble, and has more leakage than if asingle spoolie were used to feed both baffles.

Illustrated in FIGS. 3, 6, and 7 is a bifurcated impingement baffle 30disposed in the bifurcated cavity 37. The bifurcated impingement baffle30 has axially forward and aft chambers 53 and 55 with an axiallyextending axial gap 57 therebetween and the forward and aft chambers 53and 55 are disposed in the forward and aft cavities 41 and 43,respectively. The bifurcated impingement baffle 30 has a plenum chamber105 enclosed within a plenum chamber enclosure 108, illustrated as adome, which is designed to receive cooling air 80 through a singlecooling air inlet 114 to the plenum chamber from an annular plenum 112between the nozzle segments 10 and the annular casing 14 which areillustrated in FIG. 1. A single spoolie 110 is disposed in the coolingair inlet 114.

Again referring to FIGS. 3, 6, and 7, the plenum chamber enclosure 108is mounted on a sealing plate 109 and to which is mounted the forwardand aft chambers 53 and 55. Cooling air passes and may be meteredthrough forward and aft inlet apertures 131 and 133 in the sealing plate109, respectively. The cooling air is flowed through the forward and aftinlet apertures 131 and 133 into the forward and aft chambers 53 and 55,respectively. Forward and aft end plates 113 and 115 cap radially innerends 111 of the forward and aft chambers 53 and 55. The forward endplate 113 has an outlet aperture 117 through which is disposed aninterstage seal cavity feed tube 119, which in turn, is sealed by a tubeend cap 121. Alternatively, the interstage cavity feed tube could bedisposed through an outlet aperture in the aft end plate.

The forward and aft chambers 53 and 55 are received in the forward andaft cavities 41 and 43, respectively, and the bifurcating rib 39 isdisposed within the gap 57 between the forward and aft chambers. Theforward and aft chambers 53 and 55 can be slid into the forward and aftcavities 41 and 43 of the bifurcated impingement baffle 30 duringassembly of the second stage turbine nozzle 4. The bifurcatedimpingement baffle 30 is then brazed or welded to the nozzle segment 10around a collar 116 of the nozzle segment illustrated in FIGS. 2 and 3.The bifurcated impingement baffle 30 and the gap 57 between the forwardand aft cavities 41 and 43 allows the impingement baffle to straddle thebifurcating rib 39.

Further referring to FIGS. 3, 6, and 7, impingement holes 70 throughforward and aft baffle walls 73 and 75 of the forward and aft chambers53 and 55, respectively, are designed for impingement-cooling of theairfoil wall 29. Stand-off pads 76 on outer surfaces 78 of the forwardand aft baffle walls 73 and 75 position the bifurcated impingementbaffle 30 and the axially forward and aft chambers 53 and 55 within thebifurcated cavity 37 and the forward and aft cavities 41 and 43,respectively, to provide good impingement-cooling of the airfoil wall29. Spent vane impingement air 82 is discharged from the airfoil wall 29through film cooling holes 84 therethrough as illustrated in FIGS. 2 and8.

The bifurcating rib 39 has at least one crossover hole 58 therethroughthough a number of crossover holes 58 are in the exemplary embodiment ofthe nozzle segment 10 as illustrated in FIGS. 1 and 5. The crossoverholes 58 allow a bulk of the cooling air 80 to be impinged onto theleading edge LE, and then provide additional cooling by passing throughthe bifurcating rib 39 and exiting the airfoil 28 through turbulatedpassages 86 in the trailing edge TE of the airfoil 28 as illustrated inFIG. 3. The crossover holes 58 in the bifurcating rib 39 allow much ofthe cooling air 80 to be used to cool both the leading and trailingedges LE and TE of the airfoil 28. A radially extending radial gap 56between the bifurcated impingement baffle 30 and the bifurcating rib 39also allows the cooling air 80 to pass from the forward cavity 41 to theaft cavity 43 to cool both the leading and trailing edges LE and TE ofthe airfoil 28. In some embodiments of the nozzle segment this couldeliminate the use of the crossover holes 58. The crossover holes 58 alsoimprove producibility because they allow a single casting core to beused. Quartz rods may be used to form the crossover holes 58 and providerigidity to the core. A thermal barrier coating (TBC) may be applied allaround the airfoil 28 with better coverage and greater ease than isusually possible with multiple airfoil vane segments. The single airfoilvane segment provides flexibility to replace just a single airfoil in anengine which is beneficial such as in the case of problems with the fuelnozzles which cause hot streaks on the airfoils. These hot streakstypically will damage only one airfoil.

The nozzle segment 10 is cantilevered from the annular casing 14 by theouter band segment 24. The airfoil 28 has a high twist angle whichcauses a resultant gas load vector in the direction 63 of the resultantgas loads to fall outside a wheelbase 120 at an aft end 128 of thenozzle segment 10 at the outer band segment 24 as shown in FIG. 2. Thewheelbase 120 generally is an axially aftwardly facing load face orfaces 150 at the aft end 128 of the nozzle segment 10 at the outer bandsegment 24. The stiffening rib 60 and the bifurcating rib 39 areillustrated as being substantially centered about a center plane 49 thatfalls outside the wheelbase 120. This causes the nozzle segment 10 towant to rotate about a radial line normal to the centerline axis 6 andmakes mounting and sealing of the single airfoil nozzle segmentdifficult. Referring further to FIGS. 1 and 8, the nozzle segments 10are radially located by the forward hooks 107 at forward ends 122 of theouter band segments 24 and by clockwise and counter-clockwise opensecond and third hooks 124 and 126 on clockwise and counter-clockwiseends 138 and 140, respectively, at aft ends 128 of the outer bandsegments 24.

Referring to FIGS. 8-10, the second and third hooks 124 and 126 of eachnozzle segment 10 are illustrated as parts of an aft flange 129 at theaft end 128 of the outer band segments 24. The clockwise open secondhook 124 engages a stud 130 extending axially forward from the secondshroud support 79. The counter-clockwise open third hook 126 of anadjacent one 132 of the nozzle segments 10 engages a radially outwardlyfacing flat flange surface 142 at the clockwise end 138 of the aftflange 129 upon which the clockwise open second hook 124 is located. Theclockwise and counter-clockwise open second and third hooks 124 and 126and the stud 130 are all rectangularly-shaped. The clockwise open secondhook 124 is C-shaped and the counter-clockwise open third hook 126 is ashiplap hook and is shiplapped with clockwise end 138 of the aft flange129 and rests along the radially outwardly facing flat flange surface142.

The clockwise and counter-clockwise open second and third hooks 124 and126 are clockwise and counter-clockwise located on the nozzle segmentwith respect to a forward looking aft view but may also be from an aftlooking forward view if the resultant gas load vector and the direction63 are canted in counter-clockwise direction with respect to a forwardlooking aft view.

The nozzle segment 10 is prevented from rotating about a radial linenormal to the centerline axis 6 at least in part by a load stop 144extending radially outwardly from the outer band segment 24 and engagingan axially forwardly facing load face 21 on the counter-clockwise openthird hook 126 of the adjacent one 132 of the nozzle segments 10. Theload stop 144 counters a moment resulting from the sum of the gas loadsbeing off the wheelbase of the axially aftwardly facing load faces 150located on an aft side 152 of the aft flange 129. The entire turbinenozzle assembly will then be in equilibrium. The turbine nozzles 4 areassembled radially which allow for axial overlap of these features. Theload stop 144 and the aftwardly facing load face 150 may be preciselyproduced in the same machining set-up. This allows the axial load faceto be used as an air seal at the aft end of the nozzle with a highdegree of control over leakage. The hooks, stud, and stops may also beused on nozzle segments 10 having more than one airfoil, particularly,if the resultant gas loads to fall outside the wheelbase of the nozzlesegment 10 at the outer band segments.

The present invention has been described in an illustrative manner. Itis to be understood that the terminology which has been used is intendedto be in the nature of words of description rather than of limitation.While there have been described herein, what are considered to bepreferred and exemplary embodiments of the present invention, othermodifications of the invention shall be apparent to those skilled in theart from the teachings herein and, it is, therefore, desired to besecured in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

Accordingly, what is desired to be secured by Letters Patent of theUnited States is the invention as defined and differentiated in thefollowing claims:

What is claimed is:
 1. A bifurcated impingement baffle comprising:axially forward and aft chambers with a gap therebetween, impingementholes through forward and aft baffle walls of the forward and aftchambers respectively, a plenum chamber in fluid flow communication withthe forward and aft chambers, and a plenum chamber enclosure enclosingthe plenum chamber and attached to the forward and aft chambers.
 2. Abifurcated impingement baffle as claimed in claim 1, further comprisinga single cooling air inlet to the plenum chamber.
 3. A bifurcatedimpingement baffle as claimed in claim 2 further comprising: a sealingplate mounted between and sealed to the plenum chamber enclosure and theforward and aft chambers, and the sealing plate having forward and aftinlet apertures disposed between the plenum chamber and the forward andaft chambers respectively.
 4. A bifurcated impingement baffle as claimedin claim 3, further comprising forward and aft end plates cappingradially inner ends of the forward and aft chambers.
 5. A bifurcatedimpingement baffle comprising: axially forward and aft chambers with agap therebetween, impingement holes through forward and aft baffle wallsof the forward and aft chambers respectively, a plenum chamber in fluidflow communication with the forward and aft chambers, a single coolingair inlet to the plenum chamber, a sealing plate mounted between andsealed to the plenum chamber enclosure and the forward and aft chambers,the sealing plate having forward and aft inlet apertures disposedbetween the plenum chamber and the forward and aft chambersrespectively, forward and aft end plates capping radially inner ends ofthe forward and aft chambers, and an outlet aperture in the forward endplate and an interstage seal cavity feed tube disposed through theoutlet aperture.
 6. A bifurcated impingement baffle as claimed in claim5, further comprising a tube end cap sealing the interstage seal cavityfeed tube.
 7. A turbine nozzle segment comprising: at least one hollowairfoil extending radially between radially outer and inner bandsegments, the airfoil located between circumferentially spaced apartpressure and suction side edges of the radially outer and inner bandsegments, the airfoil having an airfoil wall with pressure and suctionsides extending axially between leading and trailing edges of theairfoil, the airfoil wall surrounding a bifurcated cavity, a bifurcatingrib extending through the bifurcated cavity and between the pressure andsuction sides of the airfoil wall dividing the bifurcated cavity intoforward and aft cavities, a bifurcated impingement baffle includingaxially forward and aft chambers with a gap therebetween, impingementholes through forward and aft baffle walls of the forward and aftchambers respectively, a plenum chamber in fluid flow communication withthe forward and aft chambers, and a plenum chamber enclosure enclosingthe plenum chamber and attached to the forward and aft chambers, and theforward and aft chambers disposed in the forward and aft cavitiesrespectively.
 8. A turbine nozzle segment as claimed in claim 7, furthercomprising a single cooling air inlet to the plenum chamber.
 9. Aturbine nozzle segment as claimed in claim 8, further comprising: asealing plate mounted between and sealed to the plenum chamber enclosureand the forward and aft chambers, and the sealing plate having forwardand aft inlet apertures disposed between the plenum chamber and theforward and aft chambers respectively.
 10. A turbine nozzle segment asclaimed in claim 8 further comprising at least one crossover hole.
 11. Aturbine nozzle segment comprising: at least one hollow airfoil extendingradially between radially outer and inner band segments, the airfoillocated between circumferentially spaced apart pressure and suction sideedges of the radially outer and inner band segments, the airfoil havingan airfoil wall with pressure and suction sides extending axiallybetween leading and trailing edges of the airfoil, the airfoil wallsurrounding a bifurcated cavity, a bifurcating rib extending through thebifurcated cavity and between the pressure and suction sides of theairfoil wall dividing the bifurcated cavity into forward and aftcavities, a bifurcated impingement baffle including axially forward andaft chambers with a gap therebetween, impingement holes through forwardand aft baffle walls of the forward and aft chambers respectively, aplenum chamber in fluid flow communication with the forward and aftchambers, the forward and aft chambers disposed in the forward and aftcavities respectively, a single cooling air inlet to the plenum chamber,a plenum chamber enclosure enclosing the plenum chamber, a sealing platemounted between and sealed to the plenum chamber enclosure and theforward and aft chambers, the sealing elate having forward and aft inletapertures disposed between the plenum chamber and the forward and aftchambers respectively, forward and aft end plates capping radially innerends of the forward and aft chambers, and an outlet aperture in theforward end plate and an interstage seal cavity feed tube disposedthrough the outlet aperture.
 12. A turbine nozzle segment comprising: asingle hollow airfoil extending radially between radially outer andinner band segments, the airfoil located between circumferentiallyspaced apart pressure and suction side edges of the radially outer andinner band segments, the airfoil having an airfoil wall with pressureand suction sides extending axially between leading and trailing edgesof the airfoil, the airfoil wall surrounding a bifurcated cavity, abifurcating rib extending through the bifurcated cavity and between thepressure and suction sides of the airfoil wall dividing the bifurcatedcavity into forward and aft cavities, a bifurcated impingement baffleincluding axially forward and aft chambers with a gap therebetween,impingement holes through forward and aft baffle walls of the forwardand aft chambers respectively, a plenum chamber in fluid flowcommunication with the forward and aft chambers, and a plenum chamberenclosure enclosing the plenum chamber and attached to the forward andaft chambers, and the forward and aft chambers disposed in the forwardand aft cavities respectively.
 13. A turbine nozzle segment as claimedin claim 12, further comprising a single cooling air inlet to the plenumchamber.
 14. A turbine nozzle segment as claimed in claim 13 furthercomprising at least one crossover hole.
 15. A turbine nozzle segment asclaimed in claim 14, further comprising: a sealing plate mounted betweenand sealed to the plenum chamber enclosure and the forward and aftchambers, and the sealing plate having forward and aft inlet aperturesdisposed between the plenum chamber and the forward and aft chambersrespectively.
 16. A turbine nozzle segment as claimed in claim 12further comprising a stiffening rib extending radially outwardly fromand along a radially outer surface of the outer band segment andextending axially and circumferentially from a pressure side forwardcorner of the outer band segment to the bifurcating rib.
 17. A turbinenozzle segment as claimed in claim 16 wherein the stiffening rib isaxially and circumferentially aligned with the bifurcating rib.
 18. Aturbine nozzle segment comprising: a single hollow airfoil extendingradially between radially outer and inner band segments, the airfoillocated between circumferentially spaced apart pressure and suction sideedges of the radially outer and inner band segments, the airfoil havingan airfoil wall with pressure and suction sides extending axiallybetween leading and trailing edges of the airfoil, the airfoil wallsurrounding a bifurcated cavity, a bifurcating rib extending through thebifurcated cavity and between the pressure and suction sides of theairfoil wall dividing the bifurcated cavity into forward and aftcavities, a bifurcated impingement baffle including axially forward andaft chambers with a gap therebetween, impingement holes through forwardand aft baffle walls of the forward and aft chambers respectively, aplenum chamber in fluid flow communication with the forward and aftchambers, the forward and aft chambers disposed in the forward and aftcavities respectively, a single cooling air inlet to the plenum chamber,at least one crossover hole, forward and aft end plates capping radiallyinner ends of the forward and aft chambers, and an outlet aperture inthe forward end plate and an interstage seal cavity feed tube disposedthrough the outlet aperture.
 19. A bifurcated impingement bafflecomprising: axially forward and aft chambers with a gap therebetween,impingement holes through forward and aft baffle walls of the forwardand aft chambers respectively, a plenum chamber in fluid flowcommunication with the forward and aft chambers, a single cooling airinlet to the plenum chamber, a sealing plate mounted between and sealedto the plenum chamber enclosure and the forward and aft chambers, thesealing plate having forward and aft inlet apertures disposed betweenthe plenum chamber and the forward and aft chambers respectively,forward and aft end plates capping radially inner ends of the forwardand aft chambers, and a plurality of crossover holes through thebifurcating rib.
 20. A bifurcated impingement baffle as claimed in claim19 wherein the bifurcating rib is aligned with a direction of resultantgas loads on the airfoil.
 21. A bifurcated impingement baffle as claimedin claim 20 further comprising an airfoil fillet running around ajunction between the airfoil and the outer band segment and an enlargedportion of the airfoil fillet where the bifurcating rib intersects thesuction side of the airfoil.